1. Field of the Invention
The present invention relates to a method and an apparatus for coating metal strip, particularly cold-rolled steel strip, with a coating metal which is in the molten state. The metal strip is heated and annealed in a heating furnace and subsequently the required strip temperature and surface properties are adjusted in a cooling zone. In additional steps, the strip is guided through the coating container and, behind the coating container in strip pulling direction, the thickness of the coating metal and the solidification process are controlled by means of a gas-operated stripping device
2. Description of the Related Art
After the strip has been pulled out of the coating container, the coating metal, for example, zinc, which is still liquid on the strip surface is stripped by means of the gas-operated stripping device and is conveyed back downwardly into the coating metal container. This makes it possible to adjust a defined and uniform thickness of the metal layer, for example, zinc layer, on the strip surface. It is known in the art to supply the gas of the stripping device in the cold state to the nozzle. This manner of operation is carried out primarily with nitrogen.
It is the primary object of the present invention to improve the stripping behavior of the liquid coating metal on the strip surface in order to save coating metal and to still achieve a good surface quality.
In accordance with the present invention, the operating gas for the stripping device is preheated in a partial area of the cooling zone of the heating furnace.
As a result of the present invention, the gas can now be supplied to the stripping device in the heated state for stripping the liquid coating metal. This produces an improved stripping behavior on the strip surface. This, in turn, makes possible thinner thicknesses of the coating metal and an improved surface quality. Moreover, the use of the preheated operating gas makes it possible to reduce the operating costs because the volume expansion at increased temperatures as compared to the delivery state results in a lower consumption in relation to the delivered quantity.
In accordance with a further development, preheating is carried out in a temperature range of between 20xc2x0 C. to 250xc2x0 C.
In accordance with another feature, air, nitrogen or a non-oxidizing gas are used for preheating the operating gas.
The present invention also proposes an apparatus for coating metal strip, particularly cold-rolled steel strip, with liquid coating metal, for example, zinc. The apparatus includes a heating furnace in which the metal strip can be adjusted by annealing and in a cooling zone to the temperature required for coating the strip. The apparatus further includes a coating container and a gas-operated stripping device arranged following the coating container in the strip pulling direction. The object of the present invention is met by arranging a heat exchanger within the cooling zone of the heating furnace for the metal strip, wherein the gas for the stripping device can be preheated in the heat exchanger The heated operating gas can now produce the effect of the improved stripping behavior and the thinner coating metal layers without additional operating costs by means of a quantity of heat which has been removed from the heating furnace.
The apparatus according to the present invention is further simplified by using the heat exchanger instead of one of the cooling zones in the heating furnace.
In accordance with another advantageous feature, the heat exchanger is arranged instead of the cooling zone provided at the exit of the heating furnace.
In accordance with another development, a conveying means for the preheated gas of the stripping device extends directly from the heat exchanger to the stripping device.
The conveying means for the preheated gas of the stripping device can be arranged alongside the duct extending from the heating furnace to the coating container. As a result, the apparatus of the invention does not require a significant amount of additional space.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.